Update QtWebApp to 1.8.3

nullptr redefinition for old __cplusplus versions has been removed, the rest is vanilla

third_party/QtWebApp/httpserver/httpsessionstore.h

@@ -112,6 +112,14 @@ private slots:
 
     /** Called every minute to cleanup expired sessions. */
     void sessionTimerEvent();
+
+signals:
+
+    /**
+      Emitted when the session is deleted.
+      @param sessionId The ID number of the session.
+    */
+    void sessionDeleted(const QByteArray& sessionId);
 };
 
 } // end of namespace